1. Field of the Invention
The present invention relates to an image pickup apparatus, an image pickup system, and an image pickup method that perform an image pickup operation that can synthesize image data of a wide dynamic range by alternately repeating a long exposure and a short exposure.
2. Description of the Related Art
In recent years, digital cameras have been proposed and made commercially available that are configured to acquire two images of different exposure times by performing a short exposure (SE) and a long exposure (LE), and obtain a still image of a wide dynamic range by synthesizing the two images that are obtained.
Various devices have also been proposed in recent years for obtaining this kind of wide dynamic range image with respect to moving images.
For example, Japanese Patent Application Laid-Open Publication No. 2003-46857 discloses technology that synthesizes a wide dynamic range image from an image sequence for which the image pickup times have been varied between long and short times.
A flow of image pickup operations for acquiring such a conventional wide dynamic range moving image will now be described with reference to FIG. 7 and FIG. 8. FIG. 7 is a view that illustrates an example of a moving object. FIG. 8 is a timing chart that illustrates a state of respective signals when a wide dynamic range moving image is picked up.
A vertical synchronization signal VD that serves as a basis of an image pickup timing is outputted as a pulse for each fixed vertical synchronization period x.
With respect to both an SE image and an LE image, a read-out start pulse that defines a timing for reading out image data from an image pickup device is outputted after a previously set fixed time period y has elapsed after the vertical synchronization signal VD is outputted. When the read-out start pulse is outputted, sequential output of image data starts in order, for example, from a first line of the image pickup device. After a time d has elapsed after output of the read-out start pulse, image data of a last line is outputted and reading out of the image data ends.
Since an image read-out timing is previously determined in accordance with the vertical synchronization signal VD in this manner, control of the exposure time is performed by taking a time point that precedes the read-out timing by an amount of the exposure time as a running timing of an electronic shutter es. More specifically, an SE is performed by generating an electronic shutter start pulse at a time point that precedes the read-out start pulse by an amount of an SE time b (provided, b<x). Similarly, an LE is performed by generating the electronic shutter start pulse at a time point that precedes the read-out start pulse by the amount of an LE time c (provided, b≦c≦x).
When exposure is performed at such timings, with respect to an arbitrary line, an exposure interval time that is a time period from when an SE period ends until an LE period starts is x−c, and an exposure interval time that is a time period from when the LE period ends until the SE period starts is x−b, and thus the respective exposure interval times do not match.
Therefore, for example as shown in FIG. 7, in a case of obtaining a synthesized image at a frame rate corresponding to a cycle of the vertical synchronization signal VD by synthesizing an SE image and an LE image that are temporally adjacent when images of an object are picked up that moves at a constant speed from a left side of a screen to a right side against a motionless background, when simple addition or the like that has a small processing load is adopted as synthesis processing, as shown in FIG. 8, a movement amount of the object in the synthesized image is different for each frame and therefore movement of the object is observed as an unnatural motion that is not smooth.
On the other hand, when images are synthesized by performing a prediction operation or the like so as to come closer to a natural motion, not only does image processing itself become more complex, but a necessity also arises to make an image processing algorithm for synthesizing a temporally preceding SE image and a temporally succeeding LE image and the image processing algorithm for synthesizing a temporally preceding LE image and a temporally succeeding SE image different from each other, and thus the image processing becomes more complex and a processing load increases.